Adjacent channel selectivity (ACS) is a measurement of a receiver's ability to process a desired signal while rejecting a strong signal in an adjacent frequency channel. Many communication systems operate in a band of frequencies divided into multiple channels, each channel being offset by some frequency from another. ACS is measured with respect to one of those channels, or the “assigned” channel. When receiving a signal on the assigned channel, a significant amount of interference is introduced into the receiver by adjacent channels. It is important for receivers to be able to detect the signal on the assigned channel and reject, filter or attenuate any contribution from the adjacent channels. ACS is quantified as the ratio of filter attenuation for the assigned channel frequency to the filter attenuation for an adjacent channel frequency.
Most modern communication standards specify a minimum level of rejection for those adjacent channels. The requirement is often dependent on the signal level and the offset between the assigned channel frequency and the adjacent channel frequency. For example, the 3rd Generation Partnership Project (3GPP) standard, which applies to 3G and 4G receivers, specifies 5 MHz channel separation and calls for rejection of adjacent channel interferers that are roughly 40 dB higher than the signal level.